The present invention generally relates to outer bonding tools and methods of producing semiconductor devices using the outer bonding tool, and more particularly to an outer bonding tool for a tape carrier and a method of producing semiconductor devices using such an outer bonding tool.
The number of pins of integrated circuits (ICs) and large scale integrated circuits (LSIs) in particular has increased. Hence, when packaging a semiconductor chip on the surface of a circuit substrate, electrodes of the semiconductor chip are bonded to inner leads of leads which are arranged on a tape carrier, so as to assemble the semiconductor chip to the tape carrier. On the other hand, outer leads of the tape carrier are connected to patterns of the circuit substrate. This method of packaging the semiconductor chip can easily be automated and is popularly used for this reason.
FIG. 1 shows an example of the semiconductor chip which is packaged on the surface of the circuit substrate by the above described method.
In FIG. 1, a tape carrier 3 is made of a band-shaped resin film such as a polyimide system resin tape, and rectangular device holes 31 are arranged in line in the tape carrier 3 so that semiconductor chips 2 can successively be packaged thereon. Each device hole 31 has a size slightly larger than the side of the semiconductor chip 2 in the plan view. The packaging process can roughly be divided into a tape process and a semiconductor assembling process.
In the tape process, a resin film applied with an adhesive agent is cut into a band shape, and a copper film having a thickness of 20 to 40 .mu.m is laminated on the surface of the resin film. Then, the copper film is etched to form leads 35 at each part where the device hole 31 will be formed, so that the leads 35 would extend perpendicularly to each side of the rectangular device hole 31 when the device hole 31 is formed.
Each lead 35 is made up of a linear inner lead 35A which is parallel to the X-axis (or Y-axis), a central part which extends radially, and a linear outer lead 35B which is parallel to the X-axis (or Y-axis). A tip end of the inner lead 35A is arranged at a position corresponding to an electrode of the semiconductor chip 2.
Thereafter, the resin film is etched to form the rectangular device holes 31 and support films 32. Each device hole 31 is formed so that the inner leads 35A are arranged perpendicularly to each side of the device hole 31. Each support film 32 is formed to surround the corresponding device hole 31.
Next, in the semiconductor assembling process, the semiconductor chip 2 is inserted into the device hole 31 from under the device hole 31, and the electrodes of the semiconductor chip 2 are bonded to the corresponding inner leads 35A by a thermocompression bonding using an inner bonding tool.
Thereafter, the end parts of the outer leads 35B are cut at positions adjacent to the respective sides of an outer hole, and the leads 35 are subjected to a forming process so that the outer leads 35B are formed to an approximate Z-shape.
On the other hand, a rectangular packaging region having a size approximately the same, as that of the tape carrier 3 in the plan view is imaged on the packaging surface of a circuit substrate 1 on which the tape carrier 3 is packaged by an outer lead bonding. Patterns 11 are arranged perpendicularly to each side of the rectangular packaging region of the circuit substrate 1. Elongated pads 12 are provided at terminals of the patterns 11 to be bonded to the outer leads 35B when making the outer lead bonding.
Then, the outer leads 35B are aligned to the corresponding pads 12, and the tape carrier 3 mounted with the semiconductor chip 2 is placed on the circuit substrate 1. An outer bonding tool (not shown) which is heated to 250.degree. to 300 .degree. C. is used to push the outer leads 35B against the corresponding pads 12, and the outer leads 35B and the corresponding pads 12 are bonded by a thermocompression bonding.
FIGS.2A through 2C respectively show cross sections of parts at essential stages of the conventional semiconductor assembling process described above, and FIG.3 shows an essential part of the tape carrier 3 after the outer lead bonding.
In a cross section shown in FIG.2A, an outer bonding tool 4 has an internal heater (not shown) and is controlled to a temperature of 250.degree. to 300.degree. C. when in use. This outer bonding tool 4 is made of a metal such as steel materials.
The lower surface of the outer bonding tool 4 has a rectangular shape such that each side is slightly greater than the outer dimension of the tape carrier 3. A depression (or well) is provided at the central part of this lower surface of the outer bonding tool 4. The shape of this depression resembles the outer contour of the support film 32 but is slightly greater than the outer contour of the support film 32. In addition, the depth of this depression is greater than the height of the semiconductor chip 2. Hence, a pressing surface 41 at the lower surface of the outer bonding tool 4 has a frame shape to push the top surfaces of the outer leads 35B.
On the other hand, the pads 12 arranged in the frame shape on the surface of the circuit substrate 1 has a width B1 shown in FIG.2B and a length L shown in FIG. 2C. FIG.2B is a front view in cross section and FIG. 2C is a side view in cross section. The pads 12 are arranged with a pitch P shown in FIG.2B. For example, the width B.sub.1 is 100 .mu.m, the length L is 1500 .mu.m, and the pitch P is 150 to 200 .mu.m.
The outer lead 35B of the tape carrier 3 has a width B.sub.2 shown in FIG.2B which is slightly smaller than the width B.sub.1 of the pad 12. The length of a part of the outer lead 35B which is bonded is shorter than the length L of the pad 12 (for example, 1200 .mu.m shorter). For example, the width B.sub.2 is 80 .mu.m.
In order to improve the bonding characteristic when bonding the outer lead 35B and the corresponding pad 12, a solder paste 15 is precoated on the surface of the pad 12 as shown in FIG.2B. The solder paste 15 desirably has a film thickness H of 40 .mu.m.
When bonding the outer leads 35B of the tape carrier 3, the outer leads 35B are first aligned to the corresponding pads 12, and the tape carrier 3 mounted with the semiconductor chip 2 is placed on the circuit substrate 1. Next, the outer bonding tool 4 which is heated to a temperature of 250.degree. to 300.degree. C. is lowered onto the tape carrier 3, so that the pressing surface 41 makes contact with the top surfaces of the outer leads 35B. The outer bonding tool 4 is further lowered when the solder paste 15 reflows, so as to push against the outer leads 35B with a force of 20 to 30 g per lead. The temperature of the outer bonding tool 4 is lowered in this state so that the solder paste 15 hardens. As a result, the outer leads 35B and the corresponding pads 12 are bonded by the hardened solder paste 15.
But as shown in FIGS.2B and 2C, the solder paste 15 which is precoated on the surfaces of the pads 12 does not have a flat top surface and forms a bump. In addition, the film thickness H of the solder paste 15 is inconsistent among the pads 12 and varies from 20 to 60 .mu.m although originally intended to be 40 .mu.m.
For this reason, when the outer bonding tool 4 is lowered onto the solder paste 15 and heats the same, there is a problem in that the solder paste 15 having a large film thickness H flows horizontally beyond the corresponding pad 12 as shown in FIG.3 and a solder bridge 15P is formed between two adjacent pads 12. This solder bridge 15P will short-circuit the outer leads 35B and the semiconductor chip 2 will not operate in a normal manner. FIG.3 is a front view in cross section.
On the other hand, when the solder paste 15 is melted by the outer bonding tool 4, the melted solder tends to flow downwardly due to gravity, and in this case, flows horizontally on the surface of the circuit substrate 1. For this reason, the solder paste 15 mainly connects the top surface of the pad 12 and the bottom surface of the corresponding outer lead 35B, and the solder paste 15 does not flow around the outer lead 35B onto the top surface of the outer lead 35B. As a result, the bonding between the outer lead 35B and the pad 12 is essentially achieved solely by the solder paste 15 between the bottom surface of the outer lead 35B and the top surface of the pad 12, and the outer lead 35B may easily disengage from the corresponding pad 12 due to the insufficient bonding provided by the solder paste 15.
Conventionally, the length of the pressing surface 41 is approximately the same or greater than the length of the outer lead 35B as shown in FIG.2C in order to uniformly heat the outer lead 35B and uniformly melt the solder paste 15. Consequently, the pressing surface 41 also prevents the flow of the melted solder paste 15 from reaching the top surface of the outer lead 35B, thereby causing the above insufficient bonding.